Conventionally, a cellular phone, a wireless personal digital assistant and other mobile communication units or terminals are configured to inform the user of the reception of an incoming call by selectively emitting a beep sound or a melody, or causing their housings to vibrate. One example of such mobile communication units includes a microspeaker combined with a vibrator. The vibrator includes a weight rotated by a small motor to produce vibrations. The resulting combination, however, makes it difficult to reduce the size of the unit as well as the production cost. There has recently been proposed an electromagnetic exciter or multifunction speaker designed to produce both sounds and vibrations, as disclosed in Japanese patent application publication No. 2001-239210.
Referring specifically to FIG. 4, a conventional electromagnetic exciter, as shown in Japanese patent application publication No. 2001-239210, includes a cylindrical casing 12 made of synthetic resin. A diaphragm 13 is secured to the upper peripheral edge of the casing 12. The exciter also includes a magnetic circuit device which comprises an outer yoke 16 in the form of a cylindrical cup, a cylindrical, axially magnetized permanent magnet 15, and an inner yoke 17 secured on the top of the permanent magnet 15. A voice coil 14 is attached to the diaphragm 13 and inserted into a magnetic gap formed in the magnetic circuit device. The outer yoke 16 and the inner yoke 17 are both made of a high magnetic permeability material.
The magnetic circuit device is connected to the casing 12 by a suspension spring 18. The suspension spring has a plurality of arms, each of extends between the outer yoke 16 of the magnetic circuit device and the casing 12 and is configured to resiliently suspend the outer yoke 16. This arrangement allows the magnetic circuit device to be vibrated in the axial direction of the exciter.
The outer yoke 16 has an integrally formed outer ring which serves as an additional weight 19 for reducing the natural frequency of the magnetic circuit device to a sufficient extent. It should be understood that a discrete weight may be made of a material different from that of the outer yoke 16 and may be connected to the outer yoke 16.
The permanent magnet 15 provides a magnetic flux to form a strong magnetic field in the magnetic gap defined between the peripheral side wall of the inner yoke 17 and the upper side of the outer yoke 16. With this arrangement, the diaphragm 13 is vibrated to produce acoustic sounds when an electric current having acoustic frequencies flows through the voice coil 14. The casing 12 is secured, for example, to the housing of an acoustic device.
FIGS. 5 and 6 show another conventional electromagnetic exciter wherein a weight is made of a material different from that of an outer yoke and is secured to the outer yoke. As shown better in FIG. 6, the electromagnetic exciter includes a magnetic circuit device 20 composed of an outer yoke 21 having a bottom wall and a cylindrical side wall extending upwardly from the bottom wall, a flat magnet 22 arranged within the outer yoke 21, and an inner yoke 23 mounted on the top of the magnet 22. A magnetic gap g is defined between the side wall of the outer yoke 21 and the inner yoke 23 to receive a voice coil 24. The exciter includes a casing 25. The casing 25 has an outer sleeve 25b, an inner sleeve 25c, and a flat plate 25a mounted on the upper end of the outer sleeve 25b. The voice coil 24 is secured to the flat plate 25a. The outer sleeve 25b of the casing 25 has an open bottom closed by a cover 25d. This cover may be in the form of a dust mesh or a metal plate.
As shown better in FIG. 5, a suspension 26 includes an inner ring 26a, an outer ring 26b arranged in a concentric relation to the inner ring 26a, and two arcuate arms 26c for providing a connection between the inner ring 26a and the outer ring 26b. The suspension 26 extends between the outer yoke 21 of the magnetic circuit device 20 and the casing 25. More specifically, the inner ring 26a of the suspension 26 is connected to the outer yoke 21 of the magnetic circuit device 20. The outer sleeve 25b has an inwardly extending flange. The outer ring 26b of the suspension 26 is sandwiched between the inner flange of the outer sleeve 25b and the upper end of the inner sleeve 25c. As shown in FIG. 6, an annular weight 27 has an inner diameter portion 27a fitted around the outer yoke 21. The annular weight 27 is made of a material with a high specific gravity such as tungsten. The suspension 26 may be formed by, for example stamping a blank made of spring steel.
A double-sided adhesive tape 28 is attached to the top of the flat plate 25a so as to secure the electromagnetic exciter to, for example, a cell phone housing 29. With this arrangement, vibration of the voice coil 24 is transmitted through the casing 25 to the housing 29 of the cell phone.
In the prior art exciter, the annular weight is made of a material with a high specific gravity such as tungsten. This results in an increase in the production cost of the overall electromagnetic exciter. The electromagnetic exciter, as shown, has such a limited space that the annular weight can not have a sufficient mass.
It is, therefore, an object of the present invention to provide an electromagnetic exciter which can accommodate a weight of a sufficient mass.